1. Field of the Invention
The present invention relates generally to a channel assignment method in a CDMA mobile communication system, and in particular, to a method for assigning a random access channel (RACH).
2. Description of the Related Art
With the rapid growth of the mobile communication industry, a mobile communication system capable of supporting data and image services as well as the common voice service is required. Such a mobile communication is generally called a “future mobile communication system”. The future mobile communication system commonly employs a CDMA (Code Division Multiple Access) technique, and is divided into a synchronous system and an asynchronous system, the standardization of the operations of which have been conducted separately. In particular, a European future mobile communication system is called a UMTS (Universal Mobile Telecommunication System). The standardization of the operations should define various specifications on the data and image services as well as the voice service, required in the future mobile communication system. A typical operating standard relates to channel assignment. The asynchronous UMTS or W-CDMA (Wideband CDMA) mobile communication system, which is the European future mobile communication system, uses a random access channel (RACH) and a common packet channel (CPCH) as uplink common channels. Concerning the uplink common channels for the W-CDMA mobile communication system, the RACH channel is accessed by user equipment (UE; or a mobile station in the synchronous CDMA-2000 system) when the UE has no channel connected to a UTRAN (UMTS Terrestrial Radio Access Network; or a base station in the synchronous CDMA-2000 system). In a message transmission procedure utilizing the RACH, the UE transmits a preamble using an access signature for RACH, and then, transmits a message upon receipt of an ACK signal in response to the transmitted preamble. Before transmission, the message transmitted by the UE is subjected to spreading by a selected scrambling code for the RACH.
To enable the UE to access the RACH, the UTRAN, also known as UTRAN Node-B, transmits PRACH (Physical RACH) system information available in a corresponding cell to every UE in the cell through a broadcast channel. Each UE, in the cell receives the PRACH system information message transmitted from the UTRAN over the broadcasting channel. Upon receipt of the PRACH system information message, a UE intending to transmit a message over the RACH must select one of the available scrambling codes included in the received PRACH system information message. Selecting one of the received RACH scrambling codes is performed in an RRC (Radio Resource Control) layer of the UE. The selection may also be performed even in a MAC (Medium Access Control) layer according to system implementations. Preferably, the scrambling codes to be used for the RACH must be selected such that they should be distributed uniformly or during overload control of the system.
The latest UMTS standard discloses only transmission of the PRACH system information message but does not specifically describe how the UE selects the RACH, i.e., how the UE selects a scrambling code for transmitting the PRACH system information. In addition, the latest UMTS standard does not specifically describe a plan to uniformly distribute the scrambling codes to be used for the RACH. Therefore, when the UEs select the same RACH scrambling code, a collision probability between RACH transmission messages will be increased, leading to performance degradation of the UEs and a reduction in a transmission success rate of the RACH message. In addition, the frequency collisions cause an increase in the number of accesses, thereby resulting in a reduction in a battery-run time of the UE.